Some electronic devices, for example, particular types of sensors, actuators, electronics, optics, etc., have cost and performance advantages when manufactured from semiconductor materials on a miniature scale using microsystems or MEMS technology. Such miniature electronic devices can be formed with micromechanical structures such as membranes, cantilevered beams, microbridges, tethered proof masses, microhotplates, micromirrors, etc., which are integrated with transduction mechanisms such as piezoresistors, p-n junctions, field effect transistors, piezoelectric films, etc. In order for these miniature electronic devices to perform accurately, the micromechanical structures must be fabricated with precise dimensional control.
The present invention is directed to a method of forming a device from semiconductor materials in which micromechanical structures with transduction mechanisms can be formed with dimensional precision. The method includes forming a semiconductor substrate having a p-type semiconductor region on an n-type semiconductor region. A discrete semiconductor region is formed on the p-type semiconductor region and is isolated from the n-type semiconductor region. The n-type semiconductor region is exposed to an electrolyte with an electrical bias applied between the n-type semiconductor region and the electrolyte. The n-type semiconductor region is also exposed to radiation having energy sufficient to excite electron-hole pairs.
In addition, a p-n junction reverse bias is applied between the p-type semiconductor region and the n-type semiconductor region to prevent the p-type semiconductor region and the discrete semiconductor region from etching while portions of the n-type semiconductor region exposed to the electrolyte and the radiation are etched.
In preferred embodiments, the radiation is light consisting of photons with energy greater than the band gap of the n-type semiconductor region to be etched. Etching the exposed portions of the n-type semiconductor region forms a mechanical structure from at least a portion of the p-type semiconductor region. Often, a portion of the n-type region that is beneath the p-type semiconductor region is etched. At least one of the p-type semiconductor region and the discrete semiconductor region can be formed by diffusion, implantation, deposition, and epitaxial growth. In some embodiments, the discrete semiconductor region is formed into a functional element, typically a transduction device such as a resistor or a piezoresistor. In other embodiments, the discrete semiconductor region in combination with the p-type semiconductor region forms a functional p-n junction such as a piezojunction, a photodiode, a photodetector, or a chemical sensor.
In some embodiments, a patterned radiation-opaque layer is formed over the p-type semiconductor region for blocking radiation and preventing etching of areas of the n-type semiconductor region covered by the radiation-opaque layer. Preferably, the patterned radiation-opaque layer is formed from an electrical contact on the p-type semiconductor region which is employed for applying the p-n junction reverse bias. In another embodiment, the n-type semiconductor region is exposed to radiation on a side opposite from the p-type semiconductor region. In such a case, an electrical contact is typically formed on the n-type semiconductor region on the side opposite from the p-type semiconductor region having radiation-permeable and radiation-opaque areas. The radiation-opaque areas prevent etching of areas of the n-type semiconductor region covered by the radiation-opaque areas.
Various mechanical structures are formed in accordance with the present invention as follows. A cantilevered beam of p-type material is formed by etching a cavity within the n-type semiconductor region under a portion of the p-type semiconductor region and beyond three sides thereof. In addition, a microbridge of p-type material is formed by etching a passage within the n-type semiconductor region under a portion of the p-type semiconductor region and beyond two opposite sides thereof Also, a tethered proof mass of p-type material is formed by etching a passage within the n-type semiconductor region under a patterned p-type semiconductor region. A perforated membrane of p-type material is formed by forming openings through the p-type semiconductor region to expose areas of the n-type semiconductor region under the p-type semiconductor region to the electrolyte to allow etching of a cavity in the n-type semiconductor region under the p-type semiconductor region. Furthermore, a tethered microhotplate or micromirror is formed by forming openings through the p-type semiconductor region to expose areas of the n-type semiconductor region under the p-type semiconductor region to the electrolyte to allow etching of a cavity in the n-type semiconductor region under the p-type semiconductor region.